Open your art journal or altered book to the first blank page spread. Measure the size of one-half of the page spread, and cut the decorative paper so it is the same length, but twice as wide as the page.
Cut the pieces of decorative paper to size, and fold them in half. Create a sharp crease in the paper using the bone folder to burnish the edge.
Add glue to the back of the decorative paper, as well as to the page in the book. Slide the piece of decorative paper into the crease of the book. Burnish down the decorative paper using the bone folder, ensuring all the bubbles and creases are out.
Fold the facing page in toward the center of the book so it meets with the decorative paper when it is folded in. Burnish down the edge of the fold with the bone folder.
Repeat this throughout the art journal or altered book, only use different-sized fold-outs. Create fold-outs containing envelopes on the area of the paper that extends out.